1. Field of the Invention
The present invention relates to the area of glass processing, and in particular, the production of glasses in which bubbles or porosities have been reduced or eliminated entirely.
Fluoride glasses have been employed in fabricating ultra low loss optical fibers and high energy laser windows. Optical fibers made from zirconium, barium, lanthanum, aluminum, and sodium (ZBLAN) fluorides have the potential for ultra low losses, with a theoretical lower limit of 0.01 dB/km. However, the process of melting and casting glass preforms from these materials results in the formation of bubbles in the core and at the core clad interface. Thus, fluoride glasses have yet to replace existing materials due to the problem of high extrinsic scattering in the glasses due to such bubbles. The elimination of such bubbles can provide the high quality optical materials required for ultra low loss optical fibers and high energy laser windows.
The use of fluoride glasses for ultra low loss optical fibers is limited by the transmission loss or attenuation in these fibers. Since the ultimate application of ultra low loss fibers is for long length, repeaterless communications systems, both low transmission loss and length of low loss fibers are of equal concern.
Numerous preform processing techniques have been attempted to provide ultra low loss fluoride glass fibers (Comyns (1989) Critical Reports on Applied Chemistry 27:187-92). All these techniques resulted in fibers with large extrinsic scattering losses, due primarily to bubbles in the center of the preform core and at the core-cladding interface. State of the art glass casting processes have reduced the severity of bubble formation by casting the glasses at lower temperatures to minimize contraction, yet micro-bubbles remain a problem.
The lowest loss achieved with fluoride glass optical fibers has been reported to be 0.7 dB/km for a fiber 30 meters long (Kanamore et al. (1986) Jpn. J. Appl. Phys. 25: L468-L470). Due to the short length of the fiber measured, some laboratories have questioned the accuracy of this measurement, and most researchers believe that the fiber should be at least 100 meters long to obtain accurate measurements. The lowest loss reported for a 100 meter length fluoride fiber is 2.6 dB/km (Williams et al. (1989) Extended Abstracts of the 6th International Conference on Halide Glasses, Clausthal, FRG, pages 521-25).
Fluoride glasses have been estimated to have an intrinsic loss of &lt;0.01 dB/km. The use of such glasses would therefore theoretically increase the distance light signals could be transmitted by an order of magnitude relative to present silica optical fibers, which have intrinsic losses of 0.16 dB/km. The problems in achieving the intrinsic loss values for fluoride fibers are associated with absorption from transition metal ions and rare earth metal ion impurities, and the extrinsic scattering sources which result from glass processing and fiberizing. Presently, the impurity absorptions contribute very little to the measured losses in these fibers, while the scattering losses are the dominant cause of such high optical losses. Therefore, to reduce the scattering losses in these fibers, the method of glass processing requires modification so as to eliminate the sources of defects which give rise to extrinsic scattering.
The lowest losses achieved for fluoride glass optical fibers have all been achieved via a glass melting and casting process to fabricate preforms which are then drawn into fibers. The preform making processes all involve the casting of a core glass into a slightly lower refractive index cladding glass tube. When the core glass is cast, bubbles can be entrapped at the core/cladding glass interface. Then, as the core glass solidifies, a large thermal contraction occurs, creating bubbles in the center of the core. These resultant bubbles not only scatter light, but also provide nucleation sites for crystal formation when the preform is subsequently drawn into a fiber.
While attempts have been made to remove bubbles from fluoride glasses by isothermal heat treatments, this technique has not proved successful (McNamara et al. (1987) Jour. Non-Cryst. Solids 95 & 96:625-32). Some bubbles were eliminated by these heat treatments (presumably vacuum bubbles), but most collapsed to only a minimum diameter, while others broke up into many tiny micro-bubbles. In addition, the temperatures required to remove even the vacuum bubbles were excessive, causing the glass to crystallize and slump or distort geometrically.
As far as the present inventors are aware, no documented methods exist which will produce bubble-free fluoride glass preforms or bulk fluoride glass. Also, while HIP has been used previously to provide reduce defects in the production of conventional silica glass, conventional silica glasses, because of their greater viscosity and the large spread which exists between their glass transition temperatures and their melting points, are less prone to bubbles and crystallization than are fluoride glass fibers. Thus, the ability of a procedure to prevent or reduce defects in a silica glasses is not indicative of that procedure's ability to provide similar benefits in fluoride glasses.